Tuning indicator and tuning control



jan. 24, 'R950 G. E. HANSELL 2,495,326

TUNING INDICATOR AND TUNING CONTROL ATTORNEY J, i950 G. E. HANSELL 294955325 TUNING INDICATOR AND TUNING CONTROL Filed Dec. 45 1945 3 Sheets-Sheet 2 O k u Q R* 0 Y 'k \1 b Q u) l 0W //gf/ 'fPfga-fvcy n KT E Q u, 50% Q:

g lfZfo-oe- Crm/rf@ READ/N5 OWF/afg. /f/Gf/ Heeg. FREQUENCY INVENTOR GRA/vr i Han/sfu ATTORNEY jan. Z4, T950 G. E. HANSELL 2,495,325

TUNING INDICATOR AND TUNING CONTROL Filed Dec. 5, 1945 3 Sk'xeef,L-7-Shee1l 3 Patented Jan. 24, 1956 TUNING INDICATOR AND TUNING CONTROL Grant E. Hansell, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 5, 1945, Serial No. 632,979

(Cl. Z50-40) 11 Claims. 1

This application relates to means for deriving potentials representing the condition of tuning of circuits and apparatus, and to a tuning indicator actuated by said potentials and to improved automatic frequency control means using said potentials for retuning a circuit when necessary. The application is particularly applicable to use on frequency modulated signals and was in fact developed for use on frequency shift keying signals. A system utilizing the principles of my invention is disclosed in Schock et al. U. S. application Serial #632,978, filed December 5, 1945.

In frequency shift signalling, two currents of different frequencies separated by several hundred cycles are alternatively present. The currents are keyed to represent signals and they are usually keyed symmetrically about a center frequency. Obviously, since the frequency shifts back and forth the usual metering and automatic frequency control circuits are inadequate for use in the systems. An object of my invention is to provide an improved frequency tuning meter and automatic frequency control circuits for use in frequency shift signalling systems.

A s stated above, the characteristics of the signals used in frequency shift systems are such that the usual center or mean frequency variation detecting means for detecting such variations are unsatisfactory, and a further object of my invention is to provide improved means for detecting such mean or center frequency variations, and for producing potentials characteristic of such variations in the mean frequency of the frequency shifted signals.

In describing my invention in detail, reference will be made to the attached drawings wherein Fig. 1 illustrates by circuit element and circuit connection the essential features of a device for detecting the state of tuning of a receiver excited by frequency shifted signals and a meter associated with said means for indicating the state of tuning.

Figs. 2 and 3 are curves used in describing the operation of the circuit arrangement of Fig. 1.

Fig. 4 illustrates by circuit connection and circuit element an automatic tuning control system arranged in accordance with my invention for stabilizing the tuning of circuits excited by or responsive to frequency shift signals while;

Fig. 5 (with Fig. 2) illustrates by curves operr ation of the arrangement of Fig. 4.

In Fig. 1 frequency shifted signals such as might be used in channel B of the diversity system described in U. S. application Serial #632,978,

Z discriminator and detector DB, which may also be substantially as illustrated in said application. The discriminator and detector supplies at its output, current having a direct current component which when the system is properly tuned shifts back and forth about substantially zero potential as indicated adjacent the lead running from the detector to the double diode TI. As long as the frequency shifted current supplied to DB has a center frequency corresponding to the crossing point of the discriminator characteristic which may be substantially as illustrated in Fig. 2 of the drawings, the pulsating output is symmetrically related to about zero potential. The tube TI `comprises two diodes in a full wave peak rectiiier circuit. The diode DI has as its output load impedance a resistor RI shunted by a condenser CI, while the diode D2 has as its output load impedance a resistor R2 shunted by a condenser C2.

The potential drops developed at points B and C due to rectification of the input wave are fed by resistors R3 and R4 respectively to the control grid G of a cathode follower stage including tube T2. The tube T2 has its anode connected to the positive terminal of a direct current source the negative terminal of which is grounded and has its cathode connected to ground through a cathode load resistor R5 and milliammeter MI. The tube T2 acts as a vacuum tube voltage meter of the cathode follower type. The rectifiers DI D2 are peak rectiers being connected so that the peak voltage is built up by the rectier. DI is a positive peak rectifier, D2 a negative peak rectifier.

When keying is impressed on the circuit points B and C' each charges up to a voltage corresponding to the peaks of the keying. Point B becomes plus by the amount of the peak positive keyed voltage due to current ow in resistor R! as smoothed by condenser Cl. Point C becomes minus by the amount of the peak negative keyed voltage due to current ow in resistor R2 as smoothed by condenser C2. In other words, these resistors and condenser networks are such that the average of the direct current components in the diodes DI and D2 represent average peak swings. Resistors R3 and R4 are made about equal. Thus, if points B and C are equal in voltage and opposite in potential then point N will be at zero potential, as will the grid G. 'If the keying is not centered on the discriminatorDB characteristic the potential at point B or C will grow depending upon which direction the keyed filed December 5, 1945, are fed to a frequency Wave is off-center. Then the potential at point N will grow in a corresponding manner to change the bias on the grid G. With no voltage on the input of tube T2 or point N the meter MI is adjusted to any reading by adjustment of the value of resistor R5. It is preferable to adjust the meter MI to mid-scale. This may be done by connecting the diodes to groundV byswitch S so that the potential at the point N is about zero. Then R is adjusted for center scale reading of MI. Now it can readily be seen that if the voltage at N grows plus in potential the tube T2 will pass more current and the meter reading' will increase, whereas if the potential at the point N goes minus the current through tube T2 will decrease and the reading on the meter MI will decrease. I have found that the meter reading is quite linear with changes in potential at the point N. The meter may be calibrated in cycles oftune with respect to the center frequency of the discriminator DB as shown by the curves in Fig. 3. Also the meter may be used as a visual indicator while tuning a signal to the center of the discriminator DB. Condenser C3 is added to' pro'- vide a sufficiently long time constant so that the meter will not follow the keying or other modu lation.

In the arrangement of Fig. 4 the double diode TI is as in Fig. l excited by potentials at the outputof a frequency discriminator and detector DB; The discriminator and detector may be coupled with an intermediate frequency amplifier and converter IU cooperating with an interme-l diate frequency oscillator I2 which supplies` the desired intermediate frequency output to the discriminator and detector DB. A radio frequency. amplifier and converter I3 mayprecede the I. F. amplier and converter I0. The intermediate frequency oscillator I2 may be automatically tuned as will appear hereinafter.

The diodes DI and D2 operate as described above to produce at points B and Cv positive and negative potentials respectively corresponding to' the peaks of the pulse energy at the output of the discriminator DB and at A (corresponding to p-oint N of Fig. l) a resultant potential which is zero when the discriminator center frequency is centered with respect to the frequency shifted wave and the output of the discriminator swings symmetrically about zero potential. The resistors RI and R2 and condensers CI and C2 of Fig. 1 arel in Fig. 4 replaced by resistors S and I-S, and 'land I5, and condensers I'I and-25, and I8 and 24, connected as shown in the diode direct current circuits. The point A is connected to ground by a condenser IQ and by a resistor 22 to the control grid of a reactance tube 26. The Vreactance tube 25 has its cathode grounded by a: resistor 28v and its anode coupled to its grid by a condenser 3D. The condenser and resistor 22 provide a phase shifting network which insures that the phase of the generated oscillations set up on the anode of the tube 23 by the oscillator I2 is in quadrature with respect to the phase of the oscillations impressed on the grid of this tube by the phase shift--` ing network 22 and 30.

Output from the I. F. oscillator I2 in addition to being supplied to the intermediate frequency amplifier and converter I0 is supplied to the control grid GI of a screen grid tube 36. The screen grid electrode of the tube 36 is supplied by potential through a potentiometer 43 in a voltage divider circuit including resistor 43. The potentiometer 43 may be calibrated in cycles'frequency shift as will be described hereinafter.' The anode of the tube 36 gets positive potential through resistance 44 from the same source of direct current potential supplying the screen grid and is coupled by condenser 2I to a full wave rectifier system including a tube T3. The cathode of tube 36 is coupled to ground by a cathode bias resistor 3| shunted by a bypass condenser 32 to keep the cathode of tube 36V operating at the proper direct current potential. 33 is an I'. F. bypass: condenser. The full wave rectifier diode D3 and D4 circuits are completed by the resistors I5 and I6 connecting the cathode of diode D3 and the anode of diode D4 respectively to ground. These resistors are shunted by condensers 24 and 25. Condensers 24 and 25 are for the purpose of bypassing the resistors I5 and I'G for the intermediate frequency current being fed into tube T3 and for :Jrovid'i'ng a time' constant suflicient to cause diodesv D3 and D4 to build up direct current voltage across resistors I5 and I6 equal to the peak of the diode input voltage. The condensers smooth outor remove alternating current components of intermediate frequency.r

The discriminator and detector DB is' again assumed to have a characteristic as illustrated in Fig. 2. When keying is impressed on the circuit of Fig. Ll,v points C andy B charge up-to a voltage corresponding to the peaks of the keying if points D and E are at zero potential.r The voltages at points D and E depend on whether or not the full wave rectifier T3 is excited byv output fromA frequency is centered in the discriminator the.

voltage at point zero,.whereas if the frequencyis high a plusy voltage appears at A, and if low a minus voltage appears atA. These voltages may be used to driveva. tuning control means such as, for example, altunivng Ymotor, or as illustratedY in Fig. 4,- a reactance tube whichl is in the tuning. circuit of the oscillator I2. Where a tuning motor isv used. it mayvary a reactance in the oscillator circuit. A satisfactory motor tuning arrangement wherein Ythe motor tuning is controlled in one direction fora.plusvoltage-andinthe other directionV for an 1inus voltagelis'shown invCrosby U. S. Patent #2,380,941. datedJuly 24', 1945. AnyV other tuning motor arrangement which has similar characteristics may be used.r

Now if the potentiometer 43 i's adjusted so'that the. tube 36 ampli-iles, equal negative and positive voltages are set up-at the'poinfts- D andE respecd tively. Then the resultant voltage at the point A will be as shown in curve Z of Fig. 5, for a single frequency input to the discriminator not keyed the spacing X depending upon the magnitude of. the voltagesy at they points D and E. If frequency shift vkeying is used the space X may be set equal to the total frequencyshift, then if the frequency drifts in either direction-while keying is present, it will b-e corrected bythe reactance tube or motor in the usual manner. If' the keying is interrupted and left on either mark or space nothing happens to the automatic frequency controlW circuit. If the frequency drifts in one direction it is immediately corrected o'r if it drifts inthe other direction nothing' happens as long as it stays in the region shownl by X in Fig. 5.

. The operation of the-arrangement of Fig. 4 will now be given. Assume first that the tetrode 36 S has its screening electrode potential reduced to a -point so thatk no current is flowing therethrough, and the tube is cut ol so that the potentials developed by rectifier T3 due to current iiow through resistors I5 and I6 are zero at E and D. Then assume that a single frequency at the center of the characteristic of the discriminator circuit in DB is applied to the diode TI so that no current flow takes place in diodes DI and D2 and zero potentials are developed at B and C, and as a consequence at point A. If the frequency goes higher, i. e., drifts, current flow takes place in DI and a positive potential will be developed at B. Then with plus Volts at point B and zero or ground potential at D there will be a flow of current from B through resistor 8, resistor 9, resistor 6, resistor I6 to ground, and back through resistor I5 and resistor I to point B. Due to the voltage divider action of the circuit including the elements described point A will swing in the plus direction but will not go as far plus as point B because of the voltage divider action. If resistors 8 and 9 are large as compared to resistors 6 and l and i6 and I5 the voltage at the point A may be made about half the voltage at the point B. In a similar manner it can be seen that if the frequency of the single frequency wave applied to the discriminator DB drifts in the opposite direction or gOeS IOWSY than the center of the discriminator characteristic then due to current flow in D2 a negative voltage is developed at the point C. This causes a flow of current from C through resistances S and 8, resistance l', resistance I5 to ground, and through resistances I5 and 6 back to C. The resistances enumerated in this path are again a voltage divider between the points C and E and due to this voltage divider action the potential at the point A goes negative to an amount which may be about half the potential at the point C. By this operation then it will be seen that when the potentials at D and E are about zero the control potential deriving means has a characteristic as illustrated at Y in Fig. 5.

Now still assume that a single frequency wave i-s being applied to the discriminator and that it is centered on the discriminators characteristic so that the potentials at B, C and A are about zero volts. But now assume that the tube 36 has become conductive and that the point D is made negative by a fixed amount, Say K volts, and the point E is made positive by a like fixed amount, say K volts. Then the negative Voltage at the point D tends to cause current to iiow .to the point B through resistor 6, resistor 9, resistor 8, resistor l, resistor I5, resistor I6 and back to D, while the positive voltage at vthe point E tends to cause a current to flow through the same resistors in series in the opposite direction. By the voltage divider action described above, point B is biased (by positive voltage K at E and negative voltage K at D) to a positive potential slightly less than K, and point C is biased to a negative voltage slightly less than K. Under these conditions the potential at A cannot change until the voltages applied to the diodes DI and D2 overcome these voltages at B and C (slightly less than K volts). For example, if the input frequency to the discriminator and detector DB goes higher no current flow can take place in DI until the potential at B is overcome. This could take place at point P, Fig. 5. Then a current flows as described above through theV voltage divider including resistors 1, 8, 9 and 6 to increase the Potentiale@ peint A., Y.;

The characteristic of the control voltage deriving means will be as shown by the curve Z of Fig. 5. In other words, there will be no change in potential at the point A until the potential at the input to diode DI grows to an extent sucient to overcome the potential produced at point B by the voltage applied at E due to the said voltage divider action. Now if the center frequency goes down a negative potential is developed at the diode D2 cathode but no current ow takes place until this input potential grows large enough to overcome the negative potential set up at C by the voltage applied at D. This might take place at point Q of Fig. 5, and by the above described voltagedivider action taking place in resistors 6, 9, 8 and 'I a negative potential is produced at A which grows as the frequency of the currents applied to the discriminatol` goes down. In other words, under these conditions, if points B and C are biased to voltages slightly less than K volts the characteristic Z has a portion correspending to the dimension X which is at and at which zero control potential is produced at A. However, the points A, B and C will change in potential when the frequency of the wave at discriminator/DB has changed an amount suflicient to develop an input voltage suicient to overcome the effect of the voltage at E or D. The length of thisv flat portion of the characteristic may be increased or decreased by changing the voltage applied to the points D and E. These voltages may be increased or decreased by increasing or decreasing the gain of tube 36.

New again assume that zero volts are applied at the points D and E and that the received signal is centered on the discriminator characteristic but is keyed so that a square wave keyed Y symmetrically about zero is applied to the diode full wave peak rectier Ti. Due to rectiiication in diodes DI and D2 the point B will go plus by an amount commensurate with the keying in the plus direction and the point C will go minus by the same amount. Due to the divider action taking place in the resistors 8, 9, etc., a voltage of zero potential will be produced at the point A. The time constants'of the networks (condensers il, I8, 2li, 25 and resistances 6, l, 8, 3, I5,

I6) are such that average direct current potentials depending on peak values of the keying potentials are developed at B and C. Then the system has a characteristic as indicated by curve Y of Fig. 5, and if the frequency drifts higher the potential at point B will go more plus and the potential at point C less minus and the divider action produces a plus voltage at point A which may be used as described hereinbefore to control a reactance tube or a motor for tuning purposes to thereby bring the system back into tune. In the' embodiment illustrated this is accomplished by changing the simulated reactance of the reactance tubev 26 and thereby retuning the intermediate, frequency oscillator in I2 to change the center frequency to its assigned value. If the frequency drifts lower by similar action the potential at point A will become negative and tuning will be in the opposite direction.

VNow again assume that a centered keyed wave is applied to the discriminator and that the points D and E are maintained at like negative and positive voltages respectively, resulting in point C being negative and B positive by say K volts. Also assume that the voltages at the discriminator output due to frequency shiftv keying are plus and minus K volts. Under these conditions. .n0 @bange .Will ocur in the `voltages atte@ aanwas@ they diodesby DB will grow; rectication: .willtake place ineD i; and :the .potential .atpointgBfwilLf grow or .become more.. plus, and as: a". consequence theA point Aqwilligo. plusftof..returiefi-tlnel inputto..

the ldiscrim-inatcr. Ccnversely,zif 'the-center Y ire-VA quency= oi. the -rwave appliedA to; discriminator.. DBv

goes.. down. .thei.potentialfat@point ,.Auwill. Lbecome I negative ForV thisl-ccnditinnf and?. for '.the.- com. dition.:mathel preceding paragrsmlr thefvoltage. at ther/.point fil-.will fol-low.theecurve-Y of, Fig; :.A

ducedby: the ikeying.;

forffdiferena. amounts of. frequencyA shift. plus andiminus keyngvoltages ai-'efproportional tostl'lestotalA frequency shift'over the linearfportion. off; the discriminator; Therefore, they pctentiometer t3 whichregulates f the :magnitude of the .voltages at points D. and::E-; may fbefcalibratedf in' cycles f frequency .shift .so the operator :may: setup` -.at"v points *.Dzxand'; voltages.- commensurate withithe frequency through Whichrthe currents f ed to. discriminatorf'DB fare shifted;

''hebenets derived from-thearrangement are. numerous .andicertain-ithereof,willrnow be dez.

scribedr Assume'` vthat there .is no. voltage :applied to D and E. Automatic-frequency. control 'action' Willzloe as .desired while keying. is present and markand space frequencies will'belon either'side of then frequency on which' the: discriminator characteristic is'centered: Nowv ifvthe keying should stop oneither markzor space the potential atA will change and retuningwill take place in such; a. direction, asto. move the frequency-Which was marktor spacefrequency-to the center of the discriminatcr characteristic. resumedzitewill fbeoff-:center byhalfffthe rtotal swing. and Willi-remain offe'center` until Ythe auto'n matic frequency-control can act to symmetricallyl relate they mark` and space. frequencies to-` theassigned proper, centeryfrequency so that-theyare spacedetherefrom cn opposite sides by equal freeV quency '-bandsf amount of voltage atY points `and E. Now when keyinfgf-stops onevr either mari:`r orspace ndretun-Y ingfaction; ytakes;place as long-as the, frequency;

stays Wit-hin the flatportionpfxthe curve -Z-of" and I when; ,keying isresumed :it 1ismore. likelyf-to be properly centered; Intany event; it

canape; centered; much-p fastera thanl in systems Whichdo `note provide v the compensating action describedogabove. In other wcrds,.withrthe Volt.-

ages on-D andfE'fthe voltage at Vpoint A iS-Iepre. sented by the fcurvef-Zfwhelr there isrznc keying;

and'iibyvthe curveYfwhen thereis keying;- A

changeis made from onezcurve to the other dev pendingrupon. whether or not/:keying -is present.

What is claimed is:

1. In apparatus-for-producing alow frequencypotential which-variesfabove and `below zero potential when the center frequency of frequency shifted current-varies `in combination; a discrimi nator and detector with a load impedance excited bysaid frequency shifted current for deriving di'- rect` current potentials .which vary relative to and about 'zero potential in. a manner-corresponding tovariations inthe center frequency of said currents,v tworectiers liavinaelectrodesconnected Now. it: keying is;

This defectA in systems known heretofore is overcome by putting the 1 right 8". to; said-,impedance .by direct;currenmconnectiona; and; excltedfinropposedpolarityfbyisaid directcur- I rentipotentialsna loadimpedance connected inse.,4 ries 'Withftheoutput .electrodes off jeach,r rectifier;P

A. additional' impedances ccnductively-connectedctm the.-load fsimpedances; for deriving the-algebraic; sumyofethepotentials., developed ;acr sS ,said loadf, impedances, andi 'capacitors ,-in shunt to fsaidfloadr imp edalces;A for, suppressing current components.; of a frequency liiglierthan-said 1cw;frequency.

2; ,In.;a tuning meter for frequencygmodulated; currents. combinatouff a .discriminatory-auditie:- tectorcircuit foipderiving in its output direct ninja-U rentgcompcnents ,which vary relative to y substantially; .-zerofpotenti'al inf afmanner I corresponding tofslow variations ini-the center frequency of said;V currents, a. gfullf wave peak rectifier l connected-,te -Y the output of saidfgrcuit tc-be excitedwloyl said ydif-A rectfcurrent components and-,having output load' impedaneacross;v which potentials; of opposed f, pol arityf are developed resistive -means conducf, tively-connected tosaidload impedancesforzaddgl ingsaid, oppcsedlpotentials algebraicallyto prof,4 duce @resultant-potential, and anamplifleristagcf' ot the Vcathode. follower type havinga contatory grid connected tovsaidresistive meansvto be eiivtedbyf.v saidfaddednotentialsand hai/incre Current meter=- inithecatliede circuit 3.,.. 1n apparatusl for producing` a potentialwhlch is. 4constant inthe presence -of variationsin they,- frequencyof;frequency;.shiftedv current within Va bandof frequencies which` potential grows: in ab.l solute-magnitude when said; variations exceed said band-1 lofy A'frequenciesV in-i l combination,l afdisf criminatorv and-"detector circuit connectedtd-bef,l

excitedbyfsaid frequency shifted current for fpm;

ducingin output vdirect voltages which vary rc1- V4 atiuerto'; substantially ,zerapotentiab in aimannervV corresponding. to shifts. inthe frequencyl of '.saidfa current, two rectiersconnected; inuopposed.` poflarityin a rectifier circuit :to the output fof. saidf, circuit-tube excited 'byfsaid voltagesa load con--Yr nectedlz-to theoutputof eachY rectifier to produce across said loads potentialsofcpposed polarity.-

, when' rectication.takes place; means forderiving. the resultant .off the potentials; developed inhsaid/loadst and means for biasing saidrectiers.A to.` cutoff-in thenpresenceof produced direct-.vo1t--v agesv Whichare less'than -a 'selected absolute .mag

nitude.v

4.1 In signallingfapparatus toibe used witlr-fre quency shift currents, in combination, a discrimi'A natcr and-detector circuit connected to be excited` by-saidfcurrent for der-iVingin-its output directl I current components -which Avary relative to substantially zero potential inl a manner 'correspond-i ing-tovariations in the --center frequency of said l currents, afull-'wave rectifier system connected-to the output of Tsaid circuit to be excited by said direct lcurrent components, rectifier output loads across -Which'cpotentials nof opposed polarity areA developedpmeansfor `biasingsaidrectifier system tocutol linvthe Vpresence-of :derived direct icurrent components of a-magnit-ude-below a selected value, Iar 1d'meansfor'deriving the resultant cf"- the-opposed potentialsacross `.said loads.

5; Inapparatus for producing a potentialiwhich is constant in-the presence of predetermined variations inthe-frequency of current shifted above and-below aselectedcenter frequencywhich p0, tential grows in magnitude whenfsaid variations; exceed a selected amount,` incombination, a discriminator and detectorcircuit connected `to be` excitedn by said #frequency Ishiftedv vcurrent v for -de'- riving in its output--direct--current `potentials which vary relative to substantially zero potential in a manner corresponding to shifts in the frequency of said current, two rectiers connected in opposed polarity in a rectier circuit to the output of said circuit to be excited by said direct current potentials, a load impedance connected to the output of each rectifier wherein potentials of opposed polarity are developed when rectification takes place, means for deriving the algebraic sum of the potentials developed in said load impedances, and means for biasing said rectifiers to cutoff in the presence of derived direct current potentials which do not exceed a selected absolute magnitude.

6. In an automatic tuning control system to be used with frequency shifted current, in combination, a discriminator and detector circuit connect.- ed to be excited by said current for deriving in its output voltages which vary relative to zero potential in a manner corresponding to variations in the frequency of said currents, a full wave rectifier system connected to the output of said circuit to be excited by said voltages and having output load impedances across 'which potentials of opposed polarity are developed, means for biasing said rectifier system to cutoi in the presence of derived voltages of a magnitude below a selected value, resistivel means conductively connected to said load impedances for adding-.said opposed potentials algebraically to produce a resultant potential, a tuner for controlling the frequency of said first current, and means connecting said tuner to said resistive means for controlling said tuner in accordance with said resultant potential.

'7. In a signalling system to be used with frequency shifted current, in combination, apparatus including a converter and an oscillator for changing the frequency of said current, a discriminator and detector circuit coupled to said apparatus to be excited by said frequency shifted current of changed frequency for deriving in its output voltages which vary relative to zero potential in a manner corresponding to variations in the frequency of said currents of changed frequency, a full wave rectifier system connected to the output of said circuit to be eX- cited by said voltages and having load impedances in its output across which potentials of opposed polarity are developed and having other impedances in which the opposed potentials are algebraically combined to produce a resultant potential, a reactance tube coupled to said oscillator for controlling the frequency of operation thereof, means connecting said tube to said other impedances for controlling said reactance tube in accordance with said resultant potential, means for biasing said rectifier system to cutoff in the presence of derived varying voltages of absolute magnitudes below a selected value, comprising a full wave rectifier system coupled to the output of said oscillator and having output loads wherein biasing potentials are developed, and a coupling between said loads and the recti- Iiers of said first mentioned full wave rectifier system. f

8. In apparatus for producing a potential which varies above and below zero potential when the center frequency of frequency shifted alternating currents varies in combination, a discriminator and detector with an output irnpedance, connections for impressing currents shifted in frequency by signals above and below said center frequency on said discriminator whereby direct current potentials appear across said impedance which vary relatively to and about zero potential in a manner corresponding to variations in the center frequency of said currents, two rectiiiers each having an anode and a cathode,a direct current conductive connection between said impedance and the cathode of one rectifier and the anode of the other rectifier, substantially equal current conductive impedances in series connected between the anode of said one rectifier and the cathode of the other rectifier at which impedances potentials are developed which are of opposed polarity and are a measure of the peaks of the frequency shifts of said alternating current with respect to said center frequency, connections for utilizing the differential of the potentials developed across said last-named impedances, and a capacitor in shunt to each of said load impedances for suppressing current components of repetition rate higher than the rate at which said developed potentials vary.

9. In apparatus for producing and utilizing a potential which varies above and below zero potential when the mean frequency of alternating current varies or drifts slowly in combination, a discriminator and Adetector with an output impedance, connections for impressing said .alternating current on said discriminator whereby direct current potential, Which varies relative to and about zero potential in a manner corresponding to variations in the mean frequency of said currents, appears across said impedance, two rectifier tubes each having an anode and a cathode, a direct current conductive connection between said output impedance and the cathode of one tube and the anode of the other tube, substantially equal impedances in series connected between the anode of said one tube and the cathode of said other tube, whereat opposed potentials are developed which are measures of the variations of the mean frequencies of said alternating current, a utilization circuit having input electrodes coupled to the junction point between said last named series impedances whereby the resultant of said opposed potentials fed to said utilization circuit, and connections for compensating said opposed potentials to delay production of said resultant potential until said mean frequency variations exceed a predetermined amount, comprising connections for applying a positive potential to the cathode of said other tube, and a negative potential to the anode of said one tube.

10. In apparatus for producing and utilizing a potential which varies above and below zero potential when the center frequency of frequency shifted alternating currents varies or drifts slowly in combination, a discriminator and detector with an output impedance, connections for impressing said alternating current shifted in frequency by signals above and below said center frequency on said discriminator whereby direct current potential, which varies relative to and about zero potential in a manner corresponding to variations in the center frequency of said alternating currents, appears across said impedance, two rectifier tubes each having an anode and a cathode, a direct current conductive connection between said output impedance and the cathode of one tube and the anode of the other tube, substantially equal impedances in series connected between the anode of said one tube and the cathode of said other tube whereat opposed potentials are developed which are measures of the peaks of the shifts of said alternating cur- 11 VAieifit, iautomatic Stu-ning controlffappaifatus :o2on- -neoted to "Ithe Qiiincsti'on point 'between said.` last 4vnamed'seiliesiimpedanoes:imlcontrol the frequency of -saidilternating'rourrentpotentials iin''accordanceiwith the'resultantfV fsaid'iopposedzpotenfinals, and vxconneetions lfor compensating ssaid opposed potentials to delayfprdutionfofisaid vreslt'ant potential l' until 'said 'fl-mean frequency l variations 'exoeedi aflpredeterminedramount; Icom- --pifising i eonneetions for apijlyin'g'- a`n positive .f p0 `tential to *the 'fcathode Sofi-said bother `tube, fand A=a 1' negative potentialto 'L' the 4anode @of f said one tube.

P11. In lapparatus -for '-pioducing 'fandV utilizing i potential whenifthezenterf frequency o'f frequency shifted 1' alternating currents "varies ordrfts .iin Combination, fa' dscriminator and "deteotor'fwith 'fan= otput impedance, conneetionsfor impressf'ing a current shifted -'in"frequ`enc'y lfby signals above and below said center frequencyon''said "dis'c'ifi'rinator whereby 'diret current pot'ential, -Which' varies relativef'to aid-bot'lzerolpotential inf va 1 manner 'corresponding to variations 'in the 'center frequency of- `said #alternating f'ourrents, v`appears i across said lim1 e'darice,v two rectifier. tubes I each saving "an f anode' and a cathode, fi 'a hdireot 1energy Zfrom fsaid 'roseillatoig load 'impedanees coupled with the cathd'deuofesaidfiast'onerreeti- :'er f and fthe fz-node Sofi-said liiast :other: rectiner. 9andYcomieotionsbetweem'saidllastload'fimped- I-zaunees Af arid -'said l`innesta.ntianlly equal fimpedances 'Which-f preventloperationlfof said rst named reclti'ers 4lfor 1.a selected frange :of frequency frshitts 'abolits'aid'icenter fiequency.

"GRANTl E. HANSELL.

REFERENCES CITED "Thefollowing"references" are" 'oireoor'd :inf the -`Number *Name T Date A"2,267,453 `Foster "Dec, v 23, 1941 ''2g3r33g990 VYDome -Nov. '9,` 1943 123772327 *Seeley June/ 1945 

